Sarasota Dolphin Research Program

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Stable isotope values as indicators of bottlenose dolphin foraging habits
By Peggy Ostrom PhD, Michigan State University, Nélio Barros PhD, Mote Marine Laboratory and Portland State University, Craig Stricker, PhD, USGS, Sam Rossman, PhD student, Michigan State University, and Randall Wells, PhD

Sulfur isotope values of teeth from bottlenose dolphins from Sarasota Bay, the nearshore Gulf of Mexico (Gulf) and the offshore Gulf of Mexico (Offshore).

Stable isotope analysis offers a means to explore the foraging habits (foraging location and trophic level) of bottlenose dolphins that compliments other methods used by the Chicago Zoological Society’s Sarasota Dolphin Research Program (SDRP). Stable isotopes act as natural tracers that allow us to understand the origin of organisms and materials in the environment. Different reservoirs of carbon and sulfur have unique isotope ratios and these are transferred to and permeate through the food web. For example, primary producers (e.g. seagrass and phytoplankton) have unique stable carbon isotope values. This enables us to distinguish dolphins that depend on seagrass habitats from those that rely on a phytoplankton based food web. We have been able to quantify the amount of carbon that individual dolphins derive from seagrass. Thus, these data allow us to link observational data on habitat use collected by the SDRP directly to foraging. Terrestrial and marine sulfur isotope values differ, allowing us to differentiate between near shore and offshore organisms. Nitrogen isotope values increase by a very specific amount with each trophic level in the food web and enable us to ask questions about trophic level variation. For example, as part of a marine mammal commission grant we are interested to know if trophic level changes in response to the 1995 net fishing ban or extreme red tide events such as the one that occurred in 2005.

Using stable isotope values, we hope to make several unique contributions to understanding the ecology of bottlenose dolphins. Because it is often difficult to determine the origin of stranded individuals, we investigated the ability of stable isotopes to distinguish individuals from three different population units: Sarasota Bay, the nearshore Gulf of Mexico (Gulf) and offshore Gulf of Mexico. While dolphins from Sarasota Bay were of known history, those from the Gulf and offshore were of unknown history. Sulfur isotope data on tooth collagen differentiate the three groups. Because teeth are readily available from carcasses, this approach will be helpful for determining the origin of stranded individuals in west central Florida, and this would be a particularly important contribution to understanding unusual mortality events or in identifying the origin of museum species used for retrospective analysis.

This year, Sam Rossman, has joined our team as a PhD student from the Department of Zoology, Michigan State University. Prior to starting his PhD, Sam had already won several awards that supported his work on bottlenose dolphins (e.g., Department of Zoology, Hensley Endowed Fellowship). As part of his project, Sam will be investigating annuli from Sarasota Bay dolphin teeth to look at changes in foraging habits over the lifetime of an individual. The hope is that nitrogen isotope values will assist in defining time of weaning. Because nursing offspring essentially feed at a higher trophic level than their mother, time of weaning will be indicated when d15N values reach post-weaning baseline. Although calves exhibit signs of foraging within a few months after birth, they typically associate with their mother for three to six years. We do not know when weaning ceases and the extent to which the calf is nutritionally dependent on its mother. Thus, this approach will offer new insights into mother-calf relationships. The percent contribution of mother’s milk to calf diet can be quantified using a mixing model with the d15N of the first annuli as the nursing endmember and that of the baseline as the post-weaning endmember. Ultimately, we would like to know if there is a relationship between mother’s age and nutritional investment in a calf and if some calves assimilate more milk than others. This will be the first time that stable isotope analysis will be applied to individual annuli of bottlenose dolphin teeth. Because all individuals in a population are not equivalent in their foraging behavior, and foraging behavior may contribute to survivorship, we hope these data contribute to the understanding of the population viability of Sarasota Bay bottlenose dolphins.